1. Field of the Invention
The present invention relates to a coating composition adapted for use for molded plastic articles, and a plastic lens coated with said composition.
2. Related Background Art
Molded plastic articles are widely used, exploiting the advantages of light weight, ease of working, high impact strength etc. but are associated with drawbacks of susceptibility to scratching due to insufficient hardness, susceptibility to solvents, tendency of charging so as to dust absorption, and insufficient heat resistance have been inferior to molded inorganic glass articles for the use in spectacle lenses and window panes.
For this reason it has been proposed to apply a protective coating to the molded plastic article, and many coating compositions have been proposed for this purpose. Such proposals include, for example, "a coating composition containing an organic silicon compound or a hydrolyzed product thereof as a principal component (resinous component or film forming component)" which is disclosed in the Japanese Patent Laid-open Application No. 52-11261 and which was expected to provide a hard coating similar to the inorganic coating. This composition is commercially utilized for spectacle lenses.
However, since even this composition is still unsatisfactory in scratch resistance, there has been proposed, for example, a composition additionally containing colloidal silica sol, as disclosed in the Japanese Patent Laid-open Application No, 53-111336, and this composition is also commercially used for spectacle lenses.
On the other hand, the conventional spectacle lenses have mostly been produced by in-mold polymerization of a monomer, diethyleneglycol bisallylcarbonate. The resulting lens, having a refractive index of about 1.50 which is lower than the refractive index of 1.52 of glass lenses, shows a thicker rim portion within the lens in case of spectacles for near-sighted eyes, a major drawback disliked by the users. Also in case of reading glasses, the central portion of the lens becomes thicker, and such lens has been disliked by the users for this reason.
Because of such background, there have been developed monomers of a higher refractive index than that of diethyleneglycol bisallylcarbonate, as disclosed in the Japanese Patent Laid-open Applications Nos. 55-13747, 56-166214, 57-23611 and 57-54901, and, plastic lenses of medium to high refractive index of n.sub.d =1.54-1.60 are being commercialized by several manufacturers. Also there is recently proposed a plastic lens of high refractive index exceeding 1.65.
The silica sol-containing coating composition mentioned above is also used in these lenses.
However, such silica sol-containing coating composition has had a first drawback of unaesthetic appearance, showing interference fringes on the obtained coating.
Also, on such coating on the lens, there is often formed an antireflective film, consisting of a multi-layered film of inorganic oxides based on the optical interference theory. Such antireflective film shows, for example, very pale green reflection color, and there has been a second drawback that such reflected color appears uneven, depending on the position on the lens surface.